1. Field of the Invention
The present invention relates to a device for interferential spectrometry with selective modulation. More particularly the invention relates to improvements in the device described in U.S. Pat. No. 3,924,952.
The aforesaid patent relates to a highly luminous device for interferential spectrometry, which includes an interferometer with beam-splitting by translation combined with objective lens means providing fringes in its focal plane. This device permits also the correlation or the derivation of spectra or, again, the correlation of derivatives.
2. Description of the Prior Art
A device according to the aforesaid patent is characterised in that the beam-splitter is a Sagnac interferometer in which a mirror is displacable in translation.
In such a device, the analysis of the fringes is effected by placing a periodic grating of lines parallel to the fringes and of pitch p, in their plane. This grating can vibrate periodically at the frequency f, perpendicularly to the direction of the fringes. When the pitch p is equal to the interfringe i=(.lambda.F/T), the luminous flux emerging from the grating is modulated at the frequency f and this only for the wavelength .lambda., such that (.lambda.F/T)=p. Spectral analysis of a source, or the variation of the absorption of a body as a function of a wavelength is obtained by successively bringing the interfringe corresponding to the different wavelengths contained in the source to be equal to the pitch p of the grating, which can be produced by causing notably the beam-splitting T (transversal translation) of the interferometer to vary, or the focal line F of the objective lens to vary by using a zoom.
The variations in illumination in the focal plane of this device represent the Fourier transform of the spectral distribution of the source. New, it is known that to make a correlation of two spectra it is sufficient to form the product of their Fourier transforms, which product is obtained simply in the device of the aforementioned patent by replacing the periodic grating of pitch p by a grating representing the Fourier transform of the spectrum of the substance studied or sought. U.S. Pat. No. 3,924,952 also describes increasing the sensitivity of the method of detection by correlation by effecting, not the correlation of the spectra, but the correlation of the derivatives of the spectra, thus providing a more positive technique, particularly when the spectrum characteristic of the substance is a spectrum of emission or of absorption bands. To pass very easily from the correlation of the spectra to the correlation of derivatives of spectra, it suffices to place in the plane of the grating a parabolic transmission mask.
The device of U.S. Pat. No. 3,924,952 uses preferably a Sagnac interferometer as a beam-splitter interferometer, which at present enables variable resolutions to be obtained in the visible range reaching up to about 5,000.
Applicants have found that numerous applications, especially industrial, to analysis (fluorescence of compounds, absorption, atomic absorption) only require low resolutions of the order of some hundreds; and that the high cost and relative complexity of the spectrometric arrangements of the aforementioned patent do not justify their employment in industrial operations for which such a low resolution largely suffices.